Method for liquid development of electrostatic images using conductive particles as floating electrodes



United States ABSTRACT OF THE DISCLOSURE Method for liquid development of an electrostatic image without the use of fixed electrodes and without any criticality between fixed electrodes and the electrostatic image by suspending in the liquid developer conductive particles small enough to be suspended therein but large enough not to adhere to the electrostatic image, said conductive particles thereby functioning as a floating electrode.

This invention relates to a method for the liquid development of electrostatic images and more particularly to the liquid development of electrophotographic images which method dispenses with the use of fixed electrodes and any criticality between the spacing of the fixed electrodes and the electrostatic image.

In the prior art methods of electrophotographic reproduction a gas such as air, or a highly insulating liquid such as a halogenated hydrocarbon, is generally used as the carrier for the developer. In the practice of these prior art methods of electrophotographic reproduction a base material such, for example, as paper, is coated with a photoconductive material, such, for example, as zinc oxide in an insulating binder. When it is desired to reproduce an image the base material containing the photoconductive material is electrostatically charged in any well-known manner, such, for example, as by corona discharge. The charged surface of the base material is then exposed in any well-known manner to the material that is to be copied. In this exposing process the material that is to be reproduced allows the passage of very little light, whereas the material that is not to be reproduced allows the passage of maximum light. When the electrically charged base material is exposed to light, the charge leaks off from that portion which has been exposed to light and the result is that the electrical charge is selectively leaked from the photoconductive material that has been exposed to light and therefore the charge that remains on the base material is an electrostatic image of the material to be reproduced.

The base material containing the electrostatic image is then in some manner developed by covering the base material with a toner or developer material. The toner ma terial is either suspended in a gas such as air, or in a highly insulating fluid such as a halogenated hydrocarbon. The toner material adheres to those portions of the base material having an electrostatic image and the toner is then in some way aifixed to the base material such as by the application of heat which fuses the developer material to the base material.

In these prior art methods of electrophotographic reproduction the toner or developer material is applied to the base material containing the electrostatic image, either by means of magnetic brushes or screens. The use of a magnetic brush or screen, however, is subject to numerous mechanical limitations. For example, in order to use a magnetic brush to develop large areas such as widths 48 inches and larger, powerful magnets and multiple units of complex construction are required. In addition, the use of a perforated or foraminated electrode also is subject atent to mechanical limitations, since it is difficult to achieve both dimensional stability and permeability during development, particularly in liquid development. In addition, the use of any type of rigid body results in extraneous marks on the printed surface which degrades image quality. These marks are produced by point contact and rubbing between the electrodes and the printing surface.

Another disadvantage of prior art methods of electrophotographic reproduction is that in order to obtain best results it is necessary to use a fixed electrode such as a perforated or foraminated electrode which is fixed in position over the surface to be printed. The purpose of the electrode is to concentrate the fields of flux and to make the fields of flux as perpendicular as possible to the surface that is being coated. The use of a fixed electrode has the disadvantages noted above of making it difiicult to achieve dimensional stability and also results in damage to the surface to be printed. Another disadvantage of the use of a fixed electrode is that the spacing between the electrode and the surface to be printed is critical. Thus, if it was desired to print any surface which was in any way irregular, it was dilficult to achieve a uniform coating.

In view of the foregoing the primary object of the present invention is to provide an improved method of electrophotographic immersion development which does not use a fixed electrode.

Another object of the present invention is to provide an improved method of electrophotographic immersion development which will permit uniform printing of large electrostatic images and accordingly high resolution regardless of irregularities in the surface of the area being printed.

A further object of the present invention is to provide an improved method of electrophotographic immersion development which permits the development of wide and irregular surfaces without the use of screens, brushes, electrodes or other rigid structures.

Briefly stated the objects of the present invention are accomplished by adding a volume of dense conductive particles to the insulating liquid which acts as a carrier for the developer or toners that are suspended in the insulating liquid. The conductive particles which are preferably metallic are brought into suspension by rapid agitation. The surface which contains the electrostatic image to be developed is immersed in the liquid containing the suspended conductive particles. The suspended conductive particles function as an electrode and eliminate the criticality of the spacing of the electrode from the electrostatic image since the particles by virtue of being in solution float with respect to any irregularities on the surface to be printed.

In the operation of the present invention it is first necessary that an electrostatic image be formed in any well known way. It is essential, however, to the operation of the present invention that the base material on which the electrostatic image is formed have a portion thereof which is highly conductive. One method of forming an electrostatic image is to coat one surface of a highly conductive base material by any known means in order to make the surface photoconductive. The surface of the base material containing the insulative photoconductive coating is then charged either by corona discharge or friction developed static. The electrostatic image is then formed in any desired manner such, for example, as by exposing the charged surface to the image to be reproduced thereby discharging those portions of the surface which are exposed to light. In this manner of forming the electrostatic image it is essential to the operation of the invention that the base material be highly conductive and preferably have a resistivity less than 10 ohm centimeters. Since the base material must be highly conductive in the practice of the present invention paper cannot be used unless it is moist. For example, one preferred base material for the application of the present invention is aluminum foil.

Another method of forming an electrostatic image to be developed in accordance with the present invention is to coat a conductive base material such as aluminum foil with a material that is a nonconductor but which is not photoconductive. The electrostatic image is then formed on the insulating surface by any known means, such, for example, as by a point discharge or by a cathode ray tube.

It is to be understood that the foregoing: are not meant as in any way limiting the invention since the electrostatic image may be formed in any desired manner in order to be developed by means of the present invention. The only limitation insofar as the formation of the electrostatic image is concerned is that the base material on which the electrostatic image is formed must have a surface which is conductive.

Once the electrostatic image is formed on the surface of the conductive base material having the insulative coating the electrostatic image is developed by means of a toner suspended in a liquid carrier. The liquid carrier can be any insulating fluid, such as kerosene, heptane, or a halogenated hydrocarbon, especially a fluorinated hydrocarbon such as a Freon available from E. I. du Pont de Nemours. The liquid carrier should preferably have a resistivity greater than 10 ohm centimeters. The dispersible toner may comprise pigments, colored resins, droplets, immiscible liquids, etc. The toner should preferably have a resistivity greater than 10 ohm centimeters, but the insulating carrier should have a higher volume resisitivity than the toner.

In accordance with the present invention there is dispersed in the liquid carrier a conductive material, such, for example, as iron filings. The iron filings or other conductive particles are suspended in the liquid and function as a shield or electrode, thus dispensing with the criticality of the spacing of the electrode from the surface being printed. The conductive particles are brought into suspension in the insulated liquid by rapid agitation. It is essential to the operation of the present. invention that the conductive particles be suspended in the liquid carrier and that the velocity of the liquid carrier flow be low at the imaging surface, i.e., at the surface to be printed. The preferred material to be used for the conductive particles are stainless steel filings which are preferably spheroidal metal. In addition to iron or stainless steel filings, powders of copper, zinc, lead, aluminum and other metals can be used. It is preferred that the conductive materials used have a volume and surface resistivity less than ohm centimeters and that the particle sizes be between 50 and 325 mesh in size. If the particle sizes are maintained between these limits the conductive particles are small enough to be suspended in the carrier liquid and are too large to adhere to the electrostatic image. The following are examples of formulations which can be used in the practice of the present invention.

EXAMPLE 1 Substance: Quantity Dispersible insulating pigment gram 0.02 Insulating fluid (ordorless kerosene) milliliters 1,000

Insulating fluid (tetrachloroethylene) milliliters 100 Aluminum powder grams 30 In general, a liquid composition can be made in accordance with the present invention in which toner comprises a half percent of the volume, conductive particles represent 30 percent of the volume, and the insulating fluid comprises 69 /2 percent of the volume. The bulk quantity of conductive particles in volume of slurry may range from about to ten relative to insulating fluid, the optimum quanity depending on the geometric configuration and slurry characteristics of the conductive particles. For example, conductive materials of smaller particle size are usable at lower concentrations because of improved efficiency of suspension. Thus, in order to formulate an optimum solution the size of the conductive particles should be greater than the size which can be held electrostatically and small enough to be suspended in the liquid.

In general it has been found that a solution made in accordance with the present invention contains a greater quantity of toner than is necessary for a solution containing only liquid and toner and no conductive particles. This is due to the attraction of the toner to the conductive particles. It has been found, however, that good results can be obtained for insulating fluid containing conductive particles where the concentration of toner is very low. This effect is due to the extremely rapid development of the electrostatic image in accordance with the present invention as compared to liquid developer in a carrier liquid that does not contain conductive particles. For this reason development in accordance with the present invention is applicable to high speed development.

Once a liquid composition is prepared in accordance with the present invention the surface of the base material containing the electrostatic image is brought into contact with the solution containing the conductive particles and toner in suspension. The conductive particles in solution function as an electrode but since these conductive particles are in solution any irregularity in the printing surface does not result in poor resolution due to the floating nature of the electrode which renders noncritical the spacing between the electrode and the surface to be printed.

What has been described is a method for the liquid development of electrostatic images. This is accomplished by maintaining in suspension in a conventional solution of carrier liquid and toner conductive particles which function as an electrode.

I claim:

1. A method for liquid development of an electrostatic image on a surface of a base material, said method comprising developing said electrostatic image by immersion of said image bearing surface in an electrically insulating carrier liquid having suspended therein a developer and conductive particles small enough to be suspended therein but large enough not to adhere to said electrostatic image whereby said conductive particles function as a floating electrode during deposition from said carrier liquid of said developer uniformly over the area of said electrostatic image regardless of irregularities in the surface of said base material containing said electrostatic image.

2. A method for liquid development of an electrostatic image on a surface of a base material, said method comprising developing said electrostatic image by immersion of said image bearing surface in an electrically insulating carrier liquid having suspended therein a developer and conductive particles small enough to be suspended therein but large enough not to adhere to said electrostatic image and having a resistivity less than 10 ohm centimeters whereby said conductive particles function as a floating electrode during deposition from said carrier liquid of said developer uniformly over the area of said electrostatic image regardless of irregularities in the surface of said base material containing said electrostatic image.

3. A method for liquid development of an electrostatic image on an insulating surface of a base material, the

other surface of which is conductive, said method comprising developing said electrostatic image by electrically grounding said conductive surface and immersion of said image bearing surface in an electrically insulating carrier liquid having suspended therein a developer and conductive particles small enough to be suspended therein but large enough not to adhere to said electrostatic image whereby said conductive particles function as a floating electrode during deposition from said carrier liquid of said developer uniformly over the area of said electrostatic image regardless of irregularities in the surface of said base material containing said electrostatic image.

4. A method for liquid development of an electrostatic image on an insulating surface of a base material the other surface of which is conductive, said method comprising developing said electrostatic image by electrically grounding said conductive surface and immersion of said image bearing surface in an electrically insulating carrier liquid having suspended therein a developer and grounded conductive particles small enough to be suspended therein but large enough not to adhere to said electrostatic image whereby said conductive particles function as a floating electrode during deposition from said carrier liquid of said developer uniformly over the area of said electrostatic image regardless of irregularities in the surface of said base material containing said electrostatic image.

5. A method in accordance with claim 4 wherein said conductive particles have a resistivity less than 10 ohm centimeters.

6. A method in accordance with claim 4 wherein said conductive particles are spheroidal.

7. A method in accordance with claim 4 wherein said conductive particles are greater than 50 mesh and less than 325 mesh in size.

8. A method in accordance with claim 4 wherein said insulating surface is initially photoconductive.

References Cited UNITED STATES PATENTS 2,919,247 12/1959 Allen 252-62.1 3,010,842 11/1961 Ricker 11737 3,249,088 5/1966 Ostensen 118637 OTHER REFERENCES British Patent 936,618, filed Mar. 11, 1960, patented Sept. 11, 1963, all pages.

NORMAN G. TORCHIN, Primary Examiner. J. C. COOPER, Assistant Examiner. 

